Tiny new moon found around Neptune

A diminutive nugget of a moon has been discovered lurking in the inner orbit of Neptune.

The moon, dubbed Hippocamp for the half-horse, half-fish sea monster from Greek legend, is about the size of Chicago and so faint only the powerful Hubble Space Telescope can spot it. But by examining data stretching more than a decade, researchers were able to discern its dim form from 3 billion miles away.

“Being able to contribute to the real estate of the solar system is a real privilege,” said planetary scientist Mark Showalter, the lead author of a study on the discovery published Wednesday in the journal Nature. “But it shows how much we still don’t know about the ice giants, Neptune and Uranus.”

Showalter and his colleagues suggest Hippocamp is a fragment of a larger neighboring moon called Proteus, broken off during a cataclysmic collision some 4 billion years ago.

Neptune has been explored just once in human history, with a brief flyby of the Voyager 2 spacecraft in 1989. “But there are all these interesting processes going on there, that we only got a glimpse of,” Showalter said. “Atmospheric phenomena, rings with peculiar properties ... and these collisions and breakups that formed the inner moons.”

“It’s not just a dinky little moon we’ve found,” he continued. Moons such as Hippocamp “are witnesses to the formation and evolution of the planets they orbit. In my mind, they have very interesting stories to tell.”

Showalter, a senior research scientist at the Search for Extraterrestrial Intelligence Institute in Mountain View, is something of a distant moon detective. By accumulating scores of long-exposure Hubble images, then adjusting them to account for an orbiting body’s predicted movement, he has already uncovered two new moons each around Pluto and Uranus.

The resulting images “are not pretty,” Showalter said. The planets are so overexposed they become big white blotches, and the moons at their centers are little more than pale dots. The procedure generally does not capture enough data from the moons to allow scientists to take spectra — splitting light into its component parts to reveal clues about the moons’ composition.

Hippocamp is the first new inner satellite found around the solar system’s outermost planet since the Voyager 2 flyby.